Water-insoluble hydrogel-forming polymers are materials which are capable of absorbing large quantities of fluids such as water and body waste and which are further capable of retaining such absorbed fluids under moderate pressures. These absorption characteristics of such materials make them especially useful for incorporation into absorbent articles such as disposable diapers. Harper et al; U.S. Pat. No. 3,669,103; Issued June 13, 1972 and Harmon; U.S. Pat. No. 3,670,731; Issued June 20, 1972, for example, both disclose the use of hydrogel, i.e., "hydrocolloid," materials in absorbent products.
Frequently hydrogel-forming aborbent materials comprise polymers of polymerizable unsaturated carboxylic acids or derivatives thereof, such as acrylic acid and/or alkali metal and alkyl acrylates. These polymers are rendered water-insoluble by cross-linking the carboxyl group-containing polymer chains using conventional cross-linking agents such as di- or poly-functional monomer materials. The degree of cross-linking in hydrogel and hydrogel-forming materials not only determines their water-solubility but is also an important factor in establishing two other characteristics of fluid absorbing hydrogels, i.e., absorbent capacity and gel strength. Absorbent capacity of "gel volume" is a measure of the amount of water or body fluid which a given amount of hydrogel-forming material will absorb. Gel strength relates to the tendency of the hydrogel formed from such material to deform or "flow" under an applied stress.
Hydrogel-forming materials useful as absorbents in absorbent structures and articles such as disposable diapers must have adequately high gel volume and the hydrogels formed therefrom must have adequately high gel strength. Gel volume must, of course, be sufficiently high to enable the hydrogel-forming material to absorb a significant amount of the fluid which such material encounters in the absorbent article. Gel strength must be such that the hydrogel formed does not deform and fill to an unacceptable degree the capillary void space in the absorbent structure or article, thereby inhibiting both absorbent capacity of the structure or article and fluid distribution throughout the structure or article.
One known type of hydrogel-forming material having the requisite gel volume and gel strength characteristics for use in absorbent articles is the water-absorbing starch resin disclosed in Masuda et al; U.S. Pat. No. 4,076,663; Issued Feb. 28, 1978. Such materials are prepared by graft polymerizing unsaturated monomers onto polysaccharides (such as starch or cellulose) and by cross-linking the resulting graft copolymer. While such materials are quite suitable for use as absorbents in absorbent articles, such materials must utilize starch or cellulose as an essential raw material for their preparation. It is preferred that these starch (or cellulose) materials be pretreated by heating in order to swell or gelatinize them. Given the enormous volume of raw materials which would be required for use in successfully marketed absorbent articles such as disposable diapers, it would be desirable to identify additional types of efficient hydrogel-forming absorbents which, unlike the starch resins, do not consume significant amounts of energy in their preparation and which do not depend on agricultural raw materials of potentially uncertain availability for their preparation.
Hydrogel-forming materials which essentially comprise only cross-linked polymerized unsaturated monomers, and no starch or cellulose moieties, are also known. Such materials are described, for example, in Tsubakimoto et al; U.S. Pat. No. 4,286,082; Issued Aug. 25, 1981; in Westerman; U.S. Pat. No. 4,062,817; Issued Dec. 13, 1977 and in Obayashi et al; U.S. Pat. No. 4,340,706; Issued July 20, 1982. These materials are typified by cross-linked polyacrylates which are prepared by copolymerizing acrylic acid and acrylate monomers in relatively high concentration at polymerization temperatures generally above 20.degree. C.
While these known starch-free hydrogel-forming materials can be synthesized with sufficient gel volume and gel strength characteristics to be utilized in absorbent articles, there is an additional characteristic of such materials which is unrecognized in the foregoing patents and which, in the prior art materials, tends to diminish the effectiveness of such hydrogel-forming materials as absorbents in absorbent articles. This additional characteristic concerns the level of extractable polymer material in the hydrogel-forming material. Even though the hereinbefore referenced U.S. Pat. No. 4,286,082 describes hydrogels which are said to have low "water-solubles" for safety reasons, it has been discovered that the starch-free hydrogel-forming polymers of this '082 patent, as well as other known starch-free hydrogel-forming polymers, nevertheless contain significant levels of extractable polymer material. This extractable polymer material can be leached out of the formed hydrogel structure by body fluids such as urine during the time period over which such body fluid contacts hydrogel-forming material in an absorbent article. Without being bound by theory, it is believed that such polymer material extracted by body fluid in this manner can alter both the chemical and physical characteristics of the body fluid to the extent that the fluid is more slowly absorbed and more poorly held by the hydrogel-containing absorbent article. Such a situation, of course, then contributes to undesirable leakage of body fluid from the article. On the other hand, synthesis of hydrogel-forming material in a manner which maximizes gel volume (while maintaining adequate gel strength) but which minimizes the extractable polymer content thereof, will result in improved hydrogel-forming materials which are especially useful in those absorbent articles which can be worn for relatively extended periods of time without leakage.
In view of the foregoing, it is an object of the present invention to provide improved hydrogel-forming polymer compositions which are free of starch or other polysaccharide-based polymer material but which nevertheless have desirably high gel volume and gel strength characteristics and which have acceptably low levels of extractable polymer therein.
It is a further object of the present invention to provide a process for preparing such improved hydrogel-forming polymer compositions.
It is a further object of the present invention to provide absorbent structures and articles such as disposable diapers which utilize such improved hydrogel-forming polymer materials as absorbents for body fluids.